(1) Field of the Invention
The present invention relates to a color signal correction circuit used in an apparatus for processing a color signal, and particularly, relates to a color signal correction circuit which quickly corrects a color signal according to a second-order color correction formula.
(2) Description of the Related Art
In recent years, as Desk-Top Publishing (DTP) systems, multi-media systems, etc, have been widely used, low-level computers such as personal computers have been used to process color image data. Further, the number of color copiers and color facsimiles in use is increasing, therefore, demand for color image scanners and color printers will increase.
Color image scanners which output R, G and B signals each with more than 256 gradation levels respectively and color printers which can produce more than 16 million colors, by combining inks of Cyan, Magenta, Yellow and Black are now available. When color images are reproduced by using these apparatuses, color drifts are generated due to differences between the characteristics of the respective apparatuses.
For example, in a color image scanner, an original image is separated into three primary color images of R, G and B by using corresponding color filters, and an image converter such as a CCD converts the separated color images into digital electrical signals. Because the color characteristics of the color image scanners are determined by characteristics of lighting systems, the three primary color filters and the image converters, color image scanners have different characteristics. Therefore, if the color separation signal obtained in each apparatus is directly output, the output color separation signal does not have the desired characteristics. Consequently, in the color image scanner, an operation to correct the obtained color separation signals to color separation signals having the desired characteristics is necessary.
Also, the color reproduction characteristics of a color printer are determined by the characteristics of inks, dot areas and overlap conditions, and color printers have different characteristics. Therefore, if images are produced by directly using input color separation signals, the desired color image cannot be obtained. Consequently, in the color printer, an operation to correct the obtained color R, G and B separation signals to the color R, G and B or C, M, Y separation signals or C, M, Y and K having the desired characteristics is also necessary.
When a color image scanner exclusively connects to a color printer, color correction is necessary in order to change an original color image to a desired color image.
Although a strict color correction can be carried out using the Neugebaure formula, an easier color correction using a masking formula is generally carried out. In a large scale system used in making plate for a printing machine, a high speed computer having a high throughput is used to carry out the operation, and the color correction is carried out, using signals from a drum, type scanner according to the Neugebaure formula or the masking formula. However, the operation time is excessively long. Therefore, it is difficult to use such a high throughput computer in a small DTP system , etc. Further, a long operation time for the color correction becomes a problem because the computer can not be used only for the color correction. Therefore, a color correction circuit which can carry out a color correction operation in a short time is required even though the color correction will be limited in some points.
Two masking formulas of a linear (first-order) masking formula and a second-order masking formula are widely used. A linear masking formula is expressed by a formula (1), and a second-order masking formula is expressed in by a formula (2). ##EQU1##
The operation of the linear masking formula is easier than that of the second-order masking formula. However, the correction by the linear masking formula is not sufficient for all purposes. Therefore, when color correction of a high degree is necessary, a second-order masking formula is used. The present invention relates to a color correction circuit which carries out color correction by using a second-order masking formula.
When the calculations for the second-order masking formula are carried out only by software, the operating time becomes excessively very long. Therefore, several kinds of color correction circuits and color correction methods are proposed.
A color correction circuit having the shortest operating time is a memory circuit in which three color separation signals are input as look-up table address signals, and previously calculated correction values are stored in the look-up table. In this circuit, color correction values are directly obtained by applying three color separation signals. However, this circuit has a problem that the memory requirement of this circuit is very large.
For example, when each color separation signal is eight bit data which can express 256 gradation levels, and the color correction value stored in the circuit is also eight bit data, the memory capacity of the color correction circuit is 256.times.256.times.256.times.3 (colors)=48 Mbyte. As the capacity of memory devices has increased in recent years, a color correction circuit having such a memory capacity may be realized. However, such a color correction circuit must be expensive. Therefore, a type of color correction circuit in which the total memory capacity can be reduced although the operation time becomes a little longer is required.
For example, Japanese Unexamined Patent Publication (Kokai) 3-66288 discloses an image processing apparatus in which values of square terms of respective color separation signals are stored in a memory to which upper one or more bits of respective color separation signals are input as address signals, and correction values corresponding to lower eliminated bits are calculated by a interpolation method.
However, the above image processing apparatus has a problem that the operation time becomes long because the operation is carried out by the interpolation method.
Further, Japanese Unexamined Patent Publication (Kokai) 2-7671 discloses a color correction apparatus which includes latches for holding respective color separation signals; memories in which color separation signals are respectively input as address signals and values of square terms of respective color separation signals are stored; memories in which pairs of two color separation signals are respectively input as address signals and product values of respective pairs of two color separation signals are stored; a memory for storing correction indices; and an accumulator. In this apparatus, a correction value can be obtained by adding values output from respective memories in response to color separation signals by the accumulator.
In this color correction apparatus, the total capacity of the memories is reduced and the operation time is comparatively short. However, this apparatus has a problem that a many kinds of memories are provided, and the accumulator is complex because a number of the outputs added by the accumulator is large.